This invention relates to surgically implantable joint prostheses and, more particularly, to a joint prosthesis that combines advantages of modular and nonmodular prostheses.
Artificial joints of the human body, including in particular knee and hip joints, have been available for 50 years or more and have been the subject of intense development for at least the last 20 years. The earliest designs provided metal-to-bone or metal-to-metal contact between the articulating surfaces of a joint. Friction and wear were significantly reduced in subsequent designs by the introduction of ultra-high molecular weight polyethylene (UHMWPE) as a load-bearing surface. For example, a typical knee joint prosthesis has a tibial component with a polyethylene load-bearing surface in contact with a metal femoral component. Early designs of this type had polyethylene cemented to the tibia, but it is conventional today to secure a polyethylene bearing to a metal base or tray which is anchored in the tibia, typically with the aid of a stem or peg extending into the medullary canal of the tibia.
Numerous methods of securing a polyethylene bearing to a metal base have been developed over the years, as exemplified by the following patents:
There are two general types of tibial components: modular and nonmodular. A nonmodular prosthesis has a bearing secured to the base during fabrication in the factory, typically by direct compression molding. A modular prosthesis has a prefabricated bearing designed to be attached to the base during surgery.
A modular prosthesis has several advantages over nonmodular prostheses, one of which is that an assortment of different prostheses, i.e., different base/bearing combinations, can be created in the operating room from a small inventory of separate bases and bearings of various sizes, shapes and other characteristics. With a modular prosthesis, an orthopedic surgeon can implant an appropriate base for the patient and then fit the patient with several trial bearings in the process of selecting an appropriate primary bearing to attach to the implanted base. Modular bearings are often readily removable, and in such cases they have the further advantage of facilitating revision surgery, which may become necessary in cases of traumatic injury or bearing surface wear, by enabling replacement of the bearing without removing the base.
There are also advantages to a nonmodular prosthesis construction, such as design simplicity due to the absence of a need for a retaining mechanism for a removable bearing, and relatively low cost. An even more significant advantage is that a nonmodular tibial component is virtually immune to micromotion at the interface between the bearing and the base.
Micromotion is very difficult to avoid with modular components due to the need for clearances between bearing and base to ensure that they fit together during assembly in the operating room. A very secure locking method may avoid the problem, but nonmodular fabrication of the prosthesis, e.g., direct compression molding of a bearing onto a base, avoids the issue. Unfortunately, a nonmodular prosthesis has heretofore made revision surgery more difficult in that the entire prosthetic component must be removed and replaced. In addition to the extra operating time involved and extraction tools required, such as described in U.S. Pat. No. 4,459,985 to McKay, removal and replacement of the base requires sacrificing the existing fixation to the bone and has associated complications, including possible bone loss or fracture and the difficulty of reestablishing solid fixation. Nevertheless, it is conventional wisdom regarding a nonmodular prosthesis, as stated in the above-referenced Smith patent, that xe2x80x9cthe bearing component cannot be changed without changing the base component.xe2x80x9d
The present invention combines advantages of modular and nonmodular designs with a nonmodular tibial prosthesis that is convertible in vivo to a modular device. The prosthesis includes a tibial base having a retainer for a modular tibial bearing, and has a nonmodular primary bearing secured to the tibial base. According to one aspect of the invention the retainer includes a raised portion with a cavity in an inwardly facing surface thereof, and according to another aspect of the invention the retainer is adapted to cooperate with an auxiliary mechanical locking element to securely retain a modular bearing on the base.
A retainer as that term is used herein is a part of the tibial base that is capable, alone or in conjunction with an auxiliary element or elements, of retaining a modular bearing in place on the base. It may be formed on the base as a one-piece or multi-piece retainer.
According to a further aspect of the invention, a mechanical release member is mounted on the tibial base in contact with the primary bearing as a part of the nonmodular tibial prosthesis designed to be actuated during revision surgery. Upon actuation, the release member forces the primary bearing off of the tibial base, after which the primary bearing and release member are removed and replaced by a modular bearing. In some cases, conversion to a modular device would be a desirable option even during the primary surgery. It is routine for surgeons to check the patent""s range of motion before and after selecting the appropriate size of tibial component and cementing the base to the tibial plateau. With a nonmodular prosthesis the surgeon has little recourse but to dislodge the cemented base and remove it and all the cement from the tibia if the final range-of-motion check is unsatisfactory. The present invention provides a desirable alternative.
A general object of the present invention is to provide an improved surgically implantable joint prosthesis.
Another object of the present invention is to provide advantages of a nonmodular joint prosthesis, including the virtual absence of micromotion, and yet allow a surgeon performing revision surgery to change to a modular bearing without disturbing the base member and thereby jeopardizing fixation.
Another object of the invention is to extend the lifetime of an artificial joint.
These and other objects and advantages of the present invention will be more apparent upon reading the following detailed description of the preferred embodiment in conjunction with the accompanying drawings.